1. Field of the Invention
The present invention relates to an automobile bumper and an injection mold for forming such an automobile bumper.
2. Description of the Prior Art
Widespread use of automobiles because of their growing effectiveness in the society has aroused serious concern over environmental problems that they are posing. The automobile industry is attempting to take various measures to reduce pollutants which automobile emissions are responsible for.
One effective effort is automobile emission control that can be achieved by improved fuel economy. The fuel economy can be improved in many ways including the reduction of the weight of various automobile parts. One of such automobile parts is a bumper which may be attached to the front or rear end of an automobile.
The bumpers serve the primary purpose of protecting other functional parts of the automobiles upon collision, and also serve as a design component. If only a desired portion of a bumper is thinned out to make the bumper lighter, then the abilities of the bumper to absorb shocks and resist heat may be impaired, or the injection mold that is used to form the bumper may not be sufficiently filled (a phenomenon known as a short shot), reducing the aesthetic quality of the bumper.
It is therefore necessary to reduce the weight of a bumper while retaining its desired functions without impairing the aesthetic appearance of the bumper.
When an elongate thin-plate workpiece such as an automobile bumper or the like is injection-molded using a multipoint gate, the formed bumper tends to suffer a short shot, weld marks, and flow marks. According to one solution, which is disclosed in Japanese laid-open utility model publication No. 62-119213, for example, a film gate having a large inlet width is employed to fill a mold cavity with a resin material.
However, when a film gate is used in molding a bumper, the film gate is subsequently cut off the molded bumper and thrown away. Therefore, the film gate should be as short as possible in order to meet the requirement for a reduced quantity of resin material which is used to mold a thin, light bumper.
To make a film gate shorter, the film gate may be limited to a certain length along the longitudinal direction of a mold cavity for molding a bumper, rather than extending fully along the longitudinal direction of the mold cavity. With such an arrangement, however, the resin material may not completely be filled in the end portions of the mold cavity which are spaced from the film gate. Specifically, the central portion of the mold cavity which corresponds to the film gate can relatively easily be filled with the resin material, whereas the end portions of the mold cavity cannot sufficiently be filled with the resin material, resulting in a short shot. Filling the resin material completely in the entire mold cavity through the shorter film gate requires that the resin material be injected under higher pressure and the mold be clamped under greater forces. As a result, the injection molding machine used is necessarily required to be large in size, and the cost of the molded bumper is increased.
Efforts have been made to design in a short period of time an optimum injection mold for injection-molding a workpiece such as an automobile bumper based on a computerized simulation process for representing the behavior of a resin material in the injection mold.
Japanese laid-open patent publication No. 2-72916, for example, disclosed a simulating system for carrying out such a computerized simulation process. The disclosed simulating system comprises a mold design calculating device for automatically effecting calculations to design a mold in response to configuration data of a product to be molded, a molding analyzing device for simulating and analyzing the behavior of a resin material within the designed mold under given conditions, an input device for supplying analytic data to the molding analyzing device, an output device for displaying analytic results, and a data base connected to the mold design calculating device, the molding analyzing device, the input device, and the output device. The analytic results are outputted as numerical control data to a numerically controlled machine which produces an actual injection mold.
The configuration data to be supplied to the mold design calculating device are predetermined data. Based on the predetermined configuration data that are supplied, the mold design calculating device calculates a required injection pressure and required mold clamping forces, and also calculates an optimum gate position and an optimum number of gates which do not cause a molding failure. When the wall thickness of a workpiece such as an automobile bumper to be injection-molded is partly varied, the conventional simulating system cannot design an optimum configuration for an injection mold for forming such a workpiece by determining a wall thickness distribution that will not increase the injection pressure and the mold clamping forces and not lower the workpiece appearance quality, and that will keep the workpiece rigidity at a desired level.
More specifically, as described above, the thickness of an automobile bumper to be injection-molded may be partly thinned out if the bumper is to be made lighter. If the thinned-out portion of the bumper is not optimally designed, however, the injection pressure or the mold clamping forces will be increased, resulting in an increase in the size of an injection molding machine, or defects such as shrink marks, warpage, or the like will be caused, lowering the commercial value of the injection-molded bumper. The conventional simulating system fails to carry out an analytic process for determining an injection mold configuration based on an optimum wall thickness distribution.
To injection-mold an automobile bumper whose wall thickness is partly varied, a minimum resin pressure required to injection-mold the bumper is established with respect to a thicker portion of the bumper to prevent the thicker portion from having shrink marks. If such a minimum resin pressure is to be maintained throughout the entire mold cavity including end portions thereof, certain portions in the mold cavity which form thinner portions of the bumper are subject to an excessive resin pressure.